Hi
The following code works in GHC 6.8.3, works in GHC 6.9.20071111, but
doesn't work in GHC HEAD.
{-# LANGUAGE GADTs, ScopedTypeVariables #-}
module Test where
data E x = E x
data Foo where
Foo :: Gadt a -> Foo
data Gadt a where
GadtValue :: Gadt (E a)
f (Foo GadtValue) = True
f _ = False
Under GHC HEAD I get the error:
GADT pattern match with non-rigid result type `t'
Solution: add a type signature
In the definition of `f': f (Foo GadtValue) = True
Adding a type signature to f fixes this problem:
f :: Foo -> Bool
But I haven't found anywhere other than the top level f to add a type
signature. When the match is in a list comprehension, it becomes much
harder. While playing further I discovered this example:
foos = undefined
g = [() | Foo GadtValue <- foos]
This code doesn't compile under GHC HEAD, but does under 6.8.3. However,
adding the type signature:
g :: [()]
Makes the code compile. This is surprising, as [()] isn't a GADT type,
so shouldn't need stating explicitly.
Any help on what these errors mean, or how they can be fixed with local
type annotations?
Thanks
Neil
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